The present invention relates to holographic displays and in particular to holographic displays which use an electrically addressable spatial light modulator to generate modulated light for projection onto an optically addressable spatial light modulator.
It is well known that a three-dimensional image may be presented by forming an interference pattern or hologram on a planer surface. The three-dimensional image is visible when the hologram is appropriately illuminated. Recently, interest has grown in so-called computer generated holograms (CGHs) which offer the possibility of displaying high quality images, which need not be based upon real objects, with appropriate depth cues and without the need for viewing goggles. Interest is perhaps most intense in the medical and design fields where the need for realistic visualisation techniques is great.
Typically, a computer generated hologram involves the generation of a matrix of data values (each data value corresponding to a light transmission level) which simulates the hologram which, might otherwise be formed on a real planer surface. The matrix is applied to an Electrically Addressable Spatial Light Modulator (EASLM) which may be, for example, a two-dimensional array of liquid crystal elements or of acousto-optic modulators. Coherent light is directed onto the EASLM using for example a laser such that the resulting output, either reflected from the EASLM or transmitted through the EASLM, is a modulated light pattern.
In order to produce a three-dimensional image of usable size and viewing angle, the EASLM typically has to have a large number of pixels, e.g. 1010. In addition, the pixels of the EASLM must be positioned relative to one another with a high degree of accuracy. The device must also be capable of modulating coherent light, e.g. produced by a laser. These requirements are extremely demanding and expensive to achieve in practice.
An alternative approach is presented in GB2330471A and is illustrated schematically in FIG. 1. This document describes a holographic display technique, which is referred to as Active Tiling(trademark), and involves the use of a relatively small EASLM 1 in combination with a relatively large Optically Addressable Spatial Light Modulator (OASLM) 2. The holographic matrix is subdivided into a set of sub-holograms, with the data for each sub-hologram being passed in turn to the EASLM 1. The EASLM 1 is illuminated from one side with incoherent light 3. The OASLM 2 comprises a sheet of bistable liquid crystal (in one example the liquid crystal is a ferroelectric liquid crystal) which is switched from a first to a second state by incident light. Guide optics 4, disposed between the EASLM 1 and the OASLM 2, cause the output of the EASLM 1 (i.e. light transmitted through the EASLM 1) to be stepped across the surface of the OASLM 2. The bistable nature of the OASLM liquid crystal means that the portion or xe2x80x9ctilexe2x80x9d 5 of the OASLM 2 onto which a sub-holographic image is projected, remembers that image until such time as the OASLM is reset by the application of an electrical voltage. It will be appreciated that, providing a reset voltage is applied only at the end of a complete scan, immediately prior to reset the OASLM 2 will have xe2x80x9cstoredxe2x80x9d in it a replica of the complete holographic matrix. The holographic display also typically comprises a large output lens, although this is not shown in FIG. 1.
The need for an OASLM 2 in the display of GB2330471A is demanding and expensive to implement in practice. Furthermore, the need for the OASLM to have memory and for it to be reset at the end of each scan, requiring as it does the use of surface electrodes, adds complexity and therefore yet more expense to the holographic display.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantages. This and other objects are achieved at least in part by providing a holographic display having an EASLM which is illuminated with coherent light and which is used to display sub-holographic images.
According to a first aspect of the present invention there is provided a holographic display comprising:
a source of coherent light;
an Electrically Addressable Spatial Light Modulator (EASLM) in the path of the light source and arranged in use to be driven successively by a set of sub-holograms which together correspond to a holographic image; and
light guiding means arranged to guide light output from the EASLM such that the sub-holograms are displayed successively in respective tiled regions of an EASLM projection surface.
The present invention takes advantage of the xe2x80x9cmemoryxe2x80x9d which is inherent in the human eye. Providing that the light output corresponding to each sub-hologram is of sufficient amplitude, an observer will remember that sub-hologram at least for the time it takes to display the entire set of sub-holograms making up the sub-holographic image. There is thus no need for an OASLM.
In certain embodiments of the invention, the sub-hologram images appear at the projection surface as though they are illuminated with a plane wave.
Preferably, said light guiding means comprises replicating optics arranged in use to replicate the light output from the EASLM so as to provide multiple images. More preferably, the light guiding means comprises an array of electronically controlled baffles or shutters disposed between the replicating optics and said EASLM projection surface, said baffles/shutters being controlled such that only that baffle/shutter, which is aligned with a tiled region of the EASLM projection surface associated with a given sub-hologram, is open when the EASLM is being driven by that sub-hologram.
Preferably, the light guiding means comprises means disposed at the EASLM projection surface, or between the EASLM and the EASLM projection surface, for causing diverging light to be redirected to provide a plane wave. More preferably, this means comprises an array of lenses or a holographic redirector disposed at or near the EASLM projection surface.
The sub-hologram images on the EASLM projection surface may produce discrete sets of wavefronts which converge on object points.
The light source used to illuminate the EASLM may comprise a single light source, or a plurality of light sources. For example, the light source may be an array of light sources disposed behind the replicating optics such that the EASLM is illuminated at the desired angle of incidence and with a wavefront of the desired form (e.g. plane/converging/diverging).
Preferably, the angle of incidence of the light upon the EASLM depends upon the spatial position, within the hologram, of a sub-hologram currently being displayed, the angle being switched in synchronisation with the sub-hologram update rate of the EASLM. More preferably, the light source comprises an array of light sources disposed between a baffle/shutter array.
According to a second aspect of the present invention there is provided a method of displaying a hologram, the method comprising:
successively driving an Electrically Addressable Spatial Light Modulator (EASLM) with a set of sub-holograms which together correspond to a holographic image;
directing coherent light onto the EASLM; and
guiding light output from the EASLM such that the sub-holograms are displayed successively in respective tiled regions of an EASLM projection surface.
According to a third aspect of the present invention there is provided a holographic display comprising:
a light source;
an Electrically Addressable Spatial Light Modulator (EASLM) in the path of the light source and arranged in use to be driven successively by a set of sub-holograms which together correspond to a holographic image;
light guiding means arranged to guide light output from the EASLM such that the sub-holograms are displayed successively in respective tiled regions of an EASLM projection surface; and
an array of lenses disposed on the output side of said EASLM projection plane, the lenses of the array being aligned with respective tiled regions.